Sensor for circuit tracer

ABSTRACT

A circuit tracer for determinating the location of a conductor, such as a wire, which is either an open or closed circuit, and which may lie underground. The tracer includes a transmitter which is connected to the conductor, a hand-held probe, and a receiver which is connected to the probe. The probe has three different sensors: an electric field sensor, primarily for locating the terminus of an open-ended conductor or for distinguishing such a wire in a bundled cable of wires; a differential electric field sensor, for determining the direction to and location of an open-ended conductor located above ground; and an inductive sensor for determining the direction to and location of a current-carrying conductor, including an open-ended conductor lying below ground. A switch selectively provides the output from one of the sensors to the receiver unit, which determines the magnitude of any signal based upon the direction the probe is pointing. By swinging the probe back and forth, and observing the received signal, the direction to and location of the conductor may be determined.

This is a continuation of application Ser. No. 07/953,647 filed Sep. 29,1992 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to electronic detection devices,and more particularly to an apparatus for tracing and locating anopen-ended conductor or a conductor forming a closed circuit, a portionof which may lie above ground and a portion of which may lie below theground.

2. Description of the Prior Art

The art is replete with techniques and devices for determining thedirection to and location of a cable, such as an insulated,current-carrying conductor. Most of these techniques involve the use ofone or more inductive sensors, such as a coil or a coil with ahigh-permeability core, which picks up the electromagnetic signalcreated by an alternating current in the conductor. See, e.g., U.S. Pat.Nos. 4,119,908; 4,134,061; 4,220,913; 4,295,095; 4,387,340; 4,427,942;4,438,389; 4,390,836; 4,520,317; 4,542,344; 4,639,674; 4,665,369;4,672,321; 4,767,237; 4,843,324; and 5,093,622. The general direction tothe conductor is indicated when a peak or null signal is detected by theinductor, depending upon its orientation; a tangential orientation givesa peak signal and a normal orientation gives a null signal. A similartechnique is used in many devices sold by Minnesota Mining andManufacturing Company (3M--assignee of the present invention), such asthe SCOTCHTRAK TK 3B/6B circuit tracers. Other measurement techniquesmay also be used under certain circumstances. For example, in U.S. Pat.No. 4,542,334, two electrodes are used to steer a device which buries anundersea cable. The electrodes are located on either side of the cable,and capacitively couple a signal to the cable, which is then detectedand is used to provide left/right guidance. The sensing of analternating current may further be enhanced by certain signal processingmethods, such as that disclosed in U.S. Pat. No. 4,942,365.

While the tracing of current-carrying conductors is thus easilyaccomplished, this is not the case for conductors which have a break,i.e., are open-ended. In such a conductor, since there is no closedelectrical path, very little current can be established in the conductor(at least when the conductor has negligible capacitive coupling to thesurrounding medium), and so typical current-sensing inductors arerelatively useless in the attempted location of such a conductor. It hasalso not been feasible to use the guidance technique of the '334 patentsince that technique presumes that the approximate location of the cableis known, the receiving coupler is placed about the cable, and the cableis located between or very near the source electrodes. When the cable isnot so located in the immediate vicinity of the electrodes, the signalcoupled to the cable from the electrodes is too weak to be successfullyprocessed to provide a left/right signal.

One device which has partially overcome these problems is described inU.S. Pat. No. 4,686,454. That device uses both inductive sensors and acapacitive sensor; the capacitive sensor is not differential, althoughit is somewhat directional since it uses an electric field sensing"guarded" electrode. A guarded electrode is simply one in which thesensing element is shielded in certain directions by another metallicplate, which is excited by a potential similar to the electrodepotential to eliminate "fringing" flux. The metallic plate acts as adriven shield since a feedback arrangement is used to supply theamplified output signal from the sensing element to the metallic plate.This device, however, suffers from the further requirement that thesignal from the capacitive sensor must be added to the signals from theinductive sensors in order to provide reliable conductor location. Thislimitation is primarily due to the inability of the single capacitivesensor to accurately determine the precise direction associated with themaximum received signal, and thus the signals from the inductive sensorsare needed to provide further orientation. Otherwise, reliance on thecapacitive sensor signal alone would easily lead to an erroneousdetermination of the conductor location. Furthermore, the combination ofthe two signals often creates output results which are confusing. Itwould, therefore, be desirable and advantageous to devise an instrumentwhich overcomes the foregoing limitations, by providing means fordetecting an open-ended conductor which combines the benefits of adirectional sensor with a differential sensor. The instrument shouldfurther have a magnetic sensor to enable it to trace the conductor whenfor various reasons the electric field sensors are shielded from theconductor.

SUMMARY OF THE INVENTION

The present invention provides an improved circuit tracer generallycomprising (i) a transmitter which applies a test signal of alternatingvoltage to energize the conductor, (ii) a probe which senses thetime-varying electric field potential surrounding the energizedconductor or, alternatively, which detects the electromagnetic fieldwhen sufficient current can be established in the conductor, and (iii) areceiver which processes the signals from the probe to provide a visualand/or audio indication of relative signal strength which is indicativeof the conductor location.

The probe preferably includes three sensors, an electric field sensor, adifferential electric field sensor, and an inductive sensor, which areexclusively selectable by a switch on the probe handle. The electricfield sensor, which preferably takes the form of a guarded electrode, isfirst used to find the general direction to and location of theconductor. The differential sensor, which takes the form of twogenerally oppositely facing electrodes, is then used to provide greaterresolution in conductor location. The electric field sensors aregenerally used when the conductor is above ground, where the varyingelectric field is easily detected. If tracing of the conductor leads toan underground path, the probe may be switched to the inductive sensor,which takes the form of an induction coil. When the portion of theconductor being traced is underground, there is much greater capacitivecoupling between the ground and the conductor than when the conductor isabove ground. Therefore, even if the conductor is open-ended, thiseffect allows a small, but sufficient, current to be carried on theconductor which is detectable by the sensitive inductive sensor.

The guarded electrodes are provided in a novel construction wherein arear metallic shield is provided on one surface of a printed circuitboard, with the sensing element on the opposite surface, and a ringshield surrounding the sensing element on the same surface. The shieldsare driven by providing a feedback circuit to supply the output of eachsensing element to its shield. All three sensors are convenientlypackaged in the head of a probe housing, the head forming anelectrically shielded box which is electrically connected to the circuitground. The induction coil is located at the center of the probe head,and the sensing element and rear shield of the electric field sensor,and the shielded box, have a plurality of slots therein to minimize theconductive areas normal to incoming magnetic flux and to reduce eddycurrents, allowing the magnetic flux to enter into the probe head and bedetected by the coil. High-gain, low-noise amplifiers are used topreserve the favorable signal-to-noise ratio obtained with the sensors.A level may also be provided on the handle portion of the probe, whichis at an angle with respect to the main extension of the probe, to allowthe operator to determine the depth of the conductor by a triangulationtechnique.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features and scope of the invention are set forth in theappended claims. The invention itself, however, will best be understoodby reference to the accompanying drawings, wherein:

FIG. 1 is a perspective view of the transmitter unit of the circuittracing system of the present invention;

FIG. 2 is a perspective view of the receiver unit of the circuit tracingsystem of the present invention;

FIG. 3 is a perspective view of the probe unit of the circuit tracingsystem of the present invention;

FIG. 4 is a perspective view of the probe electronics, including thesensor array;

FIGS. 5A, 5B and 5C are front, side, and rear elevational views,respectively, of the novel guarded electrodes used in the electric fieldand differential sensors of the probe electronics;

FIG. 6 is a block diagram of the transmitter electronics;

FIG. 7 is a block diagram of the probe electronics;

FIG. 8 is a schematic diagram illustrating the driven shield of theguarded electrodes used by the probe unit; and

FIG. 9 is a block diagram of the receiver electronics.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference now to the figures, and in particular with reference toFIGS. 1-3, there is depicted the circuit tracing system of the presentinvention, which is generally comprised of a transmitter unit 10 (FIG.1), a receiver unit 12 (FIG. 2), and a probe unit 14 (FIG. 3). Thecircuit tracing system is particularly suited to locate electricalconductors (wires) having a break therein at an unknown location, i.e.,open circuits, although it is equally useful with conductors in aclosed-circuit, and operates whether the conductor is above or belowground. Transmitter unit (discussed in greater detail below inconjunction with FIG. 6) provides a test signal which is applied to theconductor by means of a cable 16 having an appropriate connector 18.Transmitter unit 10 is also equipped with a second cable 20 to provide aground reference. These cables and connectors may take on variousphysical embodiments depending upon the nature of the circuit to betested. For example, if the wire to be traced were connected to astandard electrical wall outlet, cables 16 and 20 could be combined intoa single cord having a compatible standard plug. Transmitter unit 10also has an on/off switch 22, a gain control switch 24; it may furtherhave a speaker or sounder 26 for indicating the power (on/off) status orbattery condition. The components of transmitter unit 10 are allcontained in a housing 28.

Receiver unit 12 similarly includes an on/off switch 30, a gain controlknob 32, and a readout dial or meter 34 for displaying the amplitude ofthe received signal. A speaker 36 is also provided so the operator canhear the relative strength of the received signal, and another switch 38is provided to change the output of receiver unit 12 from a compressedlogarithmic scale to an expanded logarithmic scale. A connector port 40receives the cable 42 from probe unit 14. The components of receiverunit 12 are contained in a housing 44, which has attached thereto ashoulder strap 46.

Probe unit 14 is constructed of a housing 48 having a handle or gripportion 50, an arm or extension portion 52, and a head or end portion54. Housings 28, 44 and 48 are all ideally water resistant, andconstructed of any durable material, preferably a polymer such ashigh-density polyethylene (HPDE), acrylonitrile butadiene styrene (ABS),or polystyrene (PS). The overall length of housing 48 is preferablyabout 66 cm. Handle 50 has an appropriate size and shape to allow theoperator to grasp probe unit 14. Proximate handle 50 are a level vial 56(a liquid-filled tube containing an air bubble), and a switch 58. Level56 allows the operator to determine the depth of a buried cable usingtriangulation, as further explained below. Switch 58 allows the operatorto choose one of three sensors in location and tracing of the conductor,as explained further below.

Probe head 54 contains the novel sensor array shown in FIG. 4. Threesensors are provided on the printed circuit board (PCB) 60: asingle-ended electric field sensor comprising a first guarded electrode62, located at the front end of head 54; a differential electric fieldsensor comprising second and third guarded electrodes 64 and 66, locatedat the sides of head 54 and generally parallel to one another; and aninductive sensor comprising an induction coil 68 located betweenelectrodes 64 and 66, with its axis perpendicular to the face ofelectrode 62, i.e., in alignment with arm 52. Coil 68 is constructedwith a high initial permeability, low retentivity core, and has a high Qto produce the best possible signal-to-noise ratio. Preamplifiers 70 areprovided for each of the electrodes 62, 64 and 66 and coil 68. The leads72 from preamplifiers 70 are connected to an analog switch. The analogswitch is controlled by wires which traverse the length of arm 52 andare connected to the input contacts of switch 58. As explained furtherbelow, the analog switch is connected to a differential amplifier whichin turn is connected to wires in cable 42 which exit handle 50.

The construction of the guarded electrodes 62, 64 and 66 is shown inFIGS. 5A-5C. Each electrode has an electrically insulative substrate 74which is preferably formed of the same material as a printed circuitboard, i.e., an epoxy resin composite. The rear face 76 of theelectrodes has a metallic shield 78 bonded to substrate 74; rear shield78 has a plurality of slots 80 (preferably about 0.25 mm wide). Thefront face 82 of the electrodes has a metallic sensing element 84 with aplurality of similar slots, preferably parallel with slots 80, and apair of metallic borders or strips 86 surrounding sensing element 84,forming an incomplete ring shield. Element 84 and strips 86 are alsobonded directly to the surface of substrate 74. The preferred materialfor element 84 and ring and rear shields 86 and 78 is copper. Rearshield 78 has two copper-plated holes 88 therein which pass throughsubstrate 74 to provide a lead for electrical conductivity with strips86, and has another copper-plated hole 90 with an insulative borderwhich passes through substrate 74 to provide a contact for sensingelement 84. The resulting guarded electrodes are highly directional(i.e., in the direction generally perpendicular to the surface ofsensing element 84). This characteristic is termed directional becausethe magnitudes of the potentials of the sensed equipotential electricfield surfaces surrounding the energized conductor diminish withdistance from the conductor. A differential electrical field potentialsensor can only measure the difference in the potential of twoequipotential surfaces. If the sensor is aligned such that the twosensing elements both lie in one equipotential surface, the detecteddifference is zero. If the sensor is aligned such that a line from onesensing element to the other is perpendicular to an intersectingequipotential surface, the detected difference is a maximum. Thus, asthe differential sensor is rotated about any axis embedded in anequipotential surface, the detected difference will change from zerowhen the line between the two sensors is in the surface (or tangent tothe surface), to the maximum when the line is perpendicular to thesurface. In other words, the amplitude of the sensed electric fieldpotential is dependent upon its angular location with respect to thenormal of element 84. The novel use of such directional electrodes in adifferential sensor has been shown to greatly improve the spatialresolution of probe unit 14, and eliminates any need for simultaneoussensing by, e.g., an inductive sensor.

The sensor array (i.e., the space defined by PCB 60 and the componentsthereon) is partially shielded by a metallic box-like screen 91 withinprobe head 54, the screen having cutouts corresponding to the locationof electrodes 62, 64 and 66. Screen 91 is also provided with alongitudinal gap to prevent eddy currents, and is connected to thecircuit ground. The slots in rear shield 78 and sensing element 84 allowthe magnetic field lines generated by current in the conductor topenetrate head 54 to coil 68; slots need be provided only in theelectric field sensor (electrode 62) for this purpose; however, for easeof manufacture, the same slotted design is used for all three of theelectrodes 62, 64 and 66. The use of the guarded electrode array andscreen 91 yields high resolution in the location process due to ease ofprecise alignment with the normal to the electric field equipotentialsand due to the maximum decoupling obtained from earth ground.

Those skilled in the art will appreciate that the differential sensorwould still function even if the electrodes were not guarded, althoughthis would decrease their resolution. Also, it is not necessary for theelectrodes 64 and 66 to be completely parallel with one another. Forexample, the differential sensor would still theoretically be able toprovide a differential signal even if these two electrodes werecoplanar. In other words, it is only necessary to position electrodes 64and 66 at two minimally spaced apart locations in order that they beable to detect the different equipotential surfaces.

Referring now to FIGS. 6-9, the various electrical circuits in thecircuit tracing system are now explained. A block diagram of theelectronics of transmitter unit 10 is shown in FIG. 6. A crystaloscillator 92 and a divider 94 comprise the frequency source for thetransmitter. The frequency of the test signal may vary widely but, inthe disclosed embodiment, the transmitter frequency is in the range of 1kHz to 300 kHz, preferably about 4-32 kHz, and most preferably about 16kHz. The latter frequency is rarely used in other EM emission devices,and also balances the competing requirements for coupling between thesignal radiating from the conductor and the electrodes when the electricfield mode is used, versus current loading of the conductor when it isunderground. A battery 96 may be supplied to provide power to unit 10,although an external power source could alternatively be used. It isunderstood that the various components of transmitter unit 10 arepowered by battery 96 although the electrical connections between thebattery and the components is omitted for clarity; similarly, all powersupplied from battery 96 is controlled by on/off switch 22.

Divider 94 is connected, and provides audible tones, to a batterycondition checking circuit 98; if circuit 98 detects low battery power,a sounder 26 is activated. The output of oscillator 92 is also directedto a flyback control circuit 102 which provides voltage conversion tomaintain a specified maximum power output regardless of load on thecircuit, and is controlled by gain control switch 24. Flyback controlcircuit 102 includes circuitry to limit the energy stored in a flybacktransformer contained in flyback supply 106. The output of flybackcontrol circuit 102 is directed to flyback supply 106 which-convertsbattery energy to a voltage for the output power amplifier 108 such thatthe power from flyback supply 106 does not exceed an amount selected bycontrol switch 24. The regulated signal is sent to an amplifier 108, andthen to the output network 110 which is connected to cables 16 and 20.Output network 110 includes inductive and capacitive resonant circuitsto effectively couple to a wide range of resistive, inductive, andcapacitive loads, while reducing the harmonic content of the outputsignal. The transmitter output is thereby operable for impedances of 1mΩ to 1 MΩ or more. The amplitude of the test signal is preferably nomore than 50 volts for personnel safety and battery economy.

A block diagram of the electronics of probe unit 14 is shown in FIG. 7.As mentioned above, each of the electrodes 62-66 and coil 68 isconnected to one of the preamplifiers 70 which, in the preferredembodiment, are junction field-effect transistor (JFET) bufferamplifiers. The outputs of preamplifiers 70 are connected to an analogswitch 112 which is controlled by mechanical switch 58 to selectivelyprovide a single output based upon only one of the electric fieldsensor, the differential sensor, or the inductive sensor. Upon referenceto the remainder of the specification, those skilled in the art willappreciate that the differential sensor and inductive sensor could beused simultaneously; in the preferred embodiment, however, they are notso used since, as those skilled in the art will further appreciate,there is no practical advantage and or synergistic effect to thecombined use of the differential sensor and the inductive sensor and,indeed, use of switch 58 and analog switch 112 ensures that theconnection to one of the sensors is completely broken before aconnection is made to another sensor ("break before make"). Analogswitch 112 is preferably the switch commonly known as number 4053B, andis sold by many companies, including Radio Corporation of America (RCA).The output from analog switch 112 is provided to a differentialamplifier 113 which sends the signal to receiver unit 12 via cable 42.Power for the various components of probe unit 14 is supplied by thebattery in receiver unit 12, via wires in cable 42.

As further shown in FIG. 8, each of the electrodes 62, 64 and 66 have"driven" shields or guards. The output voltage from each electrode maybe maximized by reducing the effective capacitance of the electrode withrespect to ground. In the present invention, this is accomplished byenergizing the ring and rear shields at a voltage which is equal to thevoltage at the sensing element, forming the driven shield. The output ofa non-inverting amplifier 114 is connected to strips 86 and rear shield78, preventing coupling of sensing element 84 to ground through theregions occupied by either the ring shield or the narrow gap formed bysubstrate 74. In order to avoid a damaging discharge into FET amplifiers70, capacitors 115 and 117 preferably has a capacitance in the range of10-10,000 picofarads.

Introducing any conductive object near the energized conductor affectsthe shape of the equipotential electric field surfaces. It is thusdesirable when measuring the electric field to disturb it as little aspossible. The driven guarded electrode minimizes the introduction ofsuch a disturbance since it aligns the potential of the guard to that ofthe sensing electrode and therefore close to the potential of theequipotential electric field surface in which it lies. The drivenguarded electrode also allows the input impedance of the electrode to behigher than if it were not driven, resulting in less disturbance of theequipotential surface. For these reasons, a driven guarded electrode issuperior to non-driven, guarded electrodes, yielding the greatestpossible signal prior to amplification, and further eliminating sensingof any voltages from the circuitry in probe head 54. The use of highgain, low noise amplifiers 70, along with the driven shield on theguarded electrodes, greatly increases the sensitivity of thesingle-ended and differential sensors in tracing low-voltage or remoteconductors.

A block diagram of the electronics of receiver unit 12 is shown in FIG.9. Again, the battery connections are omitted for ease of viewing FIG.9, but it is understood that a battery is supplied for receiver unit 12in the same manner as shown for transmitter unit 10 in FIG. 6, includinga battery condition checking circuit, and the battery of receiver unit12 is controlled by switch 30. Another oscillator 6 and divider 118,tuned to the same frequency as transmitter unit 10, provide thefrequency source for receiver The output of divider 118 is directed tothe detection circuits described below, and to a phase-lock-loop (PLL)frequency synthesizer 120. The signal from probe unit 14 passes througha first variable attenuator 122, a low pass filter 124, and a secondvariable attenuator 126. Both attenuators are regulated by gain controlknob 32 and simply maintain the amplitude of the received signal in therange necessary to sufficiently observe the signal but also avoidapplying an overvoltage to the remaining circuitry, i.e., when thereceived signal is very strong. The output of attenuator 126 is directedto a switch driver 130 which drives mixer switch 132, whose input isfrom PLL 120. The resulting output of mixer switch 132, an intermediatefrequency (IF) signal, passes through another low pass filter 134 and abandpass filter which together comprise an IF amplifier.

The conditioned signal from bandpass filter 136 may be processed in manydifferent ways to provide detection of the test signal from transmitterunit 10. In the preferred embodiment, receiver unit 12 performs thepseudo-synchronous detection routine as more fully described in U.S.Pat. No. 4,942,365, which determines the magnitude of the signals fromthe sensors. The sinusoidal signal from bandpass filter 136 is used asan input to another switch driver 138 which drives two synchronousdetectors 140 and 142. Each of the synchronous detectors includes aninverter and an analog switch, the analog switch having two inputs, onebeing the unmodified signal from switch driver 138, and the other beingthe inverted form of that signal. In detector 140, the analog switch isresponsive to the reference signal from divider 118; in detector 142,the analog switch is responsive to the reference signal from divider 118with a 90° phase shift. The two signals from detectors 140 and 142 passthrough Bessel low pass filters 144 and 146, respectively, and are thencombined in a pseudosynchronous, or chopper analog, switch 148, which isalso responsive to the reference signal from divider 118. Thepseudosynchronous signal is directed to an RMS detector 150, whichpasses the signal level to an amplifier 152. The output of amplifier 152may be based on a compressed or expanded logarithmic scale dependingupon the setting of switch 38. The output is directed to both meter 34and speaker 36.

Operation

Operation of the circuit tracing system of the present invention beginsby attaching the signal cable 20 of transmitter unit 10 to theaccessible portion of the conductor, and attaching ground cable 18 to alocal ground. Those skilled in the art will appreciate that the signalmay be applied inductively if it is impossible or undesirable todirectly (conductively) apply the signal. On/off switch 22 is turned onand, if the battery check is acceptable, gain control switch 24 isadjusted according to the particular conditions, i.e., "low" for shortrange tracing, "high" for long range. The voltage applied by thetransmitter results in the alternate charging and discharging of thecable, creating a time-varying electric field potential which can bedetected by electrodes 62, 64 and 66. Furthermore, if the conductor ispart of a closed circuit, an alternating current will be establishedwhich creates an electromagnetic field detectable by the inductivesensor. Cable 42 of probe unit 14 is plugged into connector 40 ofreceiver unit 12, and on/off switch 30 is turned on. Again, assuming thebattery checks out alright, gain control knob 32 is adjusted accordingto the conditions. If the general location of the conductor is known,switch 38 may be moved to the expanded logarithmic setting which givesthe sharpest null and best resolution in location of a cable, but if theprobe is being used at a location fairly distant from the transmitterand the location and depth of the cable are uncertain, then the operatormay want to start with switch 38 at the compressed logarithmic settingto get a general feel for the direction of the conductor.

The operator may want to begin tracing with switch 58 of probe unit 14in either the setting corresponding to the inductive sensor, or thesetting corresponding to the single-ended sensor. Use of thesingle-ended sensor allows the user to confirm that the system isoperating properly. This is accomplished by moving probe unit 14 closeto, and pointing at, the transmitter lead 16; if a signal is notimmediately detected, then the equipment should be checked for apossible malfunction. If the signal is detected, then the operator willmove probe 14 to the general area where the conductor is to be located.The single-ended sensor is then used to find the general direction toand location of the conductor. Switch 58 may thereafter be set to eitherthe differential sensor or the inductive sensor, depending upon thespecific conditions.

Notwithstanding the foregoing, the inductive sensor may instead be usedfirst if the conductor forms a closed-loop or immediately extendsunderground. Even if the conductor is open and above ground, if thestarting location for tracing is relatively close to the transmitter,then the conductor may carry enough current for a short distance toallow tracing with the inductive sensor and, if so, this form ofdetection will be more accurate than the single-ended electric fieldsensing. Switch 58 may thereafter be moved to the differential sensorsetting when the electromagnetic signal weakens. The single-endedelectric field sensor (electrode 62) is normally used only to find onewire among a bundled cable of wires, or for pinpointing the terminus ofan open-ended conductor.

Regardless of which sensor is used, location proceeds by following thepath of the conductor while swinging the probe transversely to theconductor (i.e., left-right-left). Arm extension 52 of probe 14 shouldbe held at an orientation generally perpendicular to the conductor. Forexample, if the conductor extends vertically in a wall, extension 52should be horizontal, but if the conductor travels horizontallyunderground, then extension 52 should be vertically pointed straight atthe ground. During the swinging movement, when the probe is pointeddirectly towards the conductor, meter 34 and/or speaker 36 will providean indication of alignment (i.e., a peak signal from the single-endedelectric field sensor, or a null signal from the differential electricfield or inductive sensors).

If the portion of the conductor being traced is underground, thesurrounding conductive mass of the earth may limit the electric field toa region very close to the conductor, making detection by electric fieldsensors difficult or impossible. In such cases, however, the highcapacitance of the conductor to earth allows a small current to floweven in an open conductor, and the electromagnetic field establishedthereby can be sensed by coil 68 (when oriented in the properdirection). There are cases where the electric field sensors arepreferred for underground conductors. If the conductor is buried shallowor is in poorly conducting soil, the technique is viable, and canfurther aid in discriminating the paths of the desired conductor evenwhen it is in the vicinity of other conductors. In such a case, thetracing signal may flow in the other conductors as well, and magneticsensing cannot easily discriminate between these other conductors andthe desired conductor, whereas the desired conductor can be adequatelydiscriminated with the differential electric field sensor. Also, whilethe terminus of a buried open conductor can be determined using theinductive sensor, if the conditions are conducive to use of the electricfield sensors, then the location of the terminus can be determined moreprecisely with the single-ended sensor.

If the conductor is underground, the operator may also want to know itsdepth. This may easily be determined by using level 56 in a well-knowntriangulation operation. Once the azimuthal location and direction ofthe conductor is known, a marker, such as a pin flag, may be placed onthe ground. The operator then moves away from the marker, in a directionperpendicular to that of the conductor path, while maintaining the probein an orientation wherein the bubble in the level remains between thetwo lines, i.e., with handle 50 at a horizontal pitch. As the operatormoves away from the conductor, meter 34 will begin to drop off,establishing a null or reference point for triangulation at the locationof the minimum signal. By measuring the distance from this referencepoint to the marker, and knowing the relative angle between handle 50and extension 52, the operator may calculate the depth of the conductor.To simplify this procedure, however, handle 50 preferably extends at anangle A of 45° with respect to extension 52. In this manner, thetriangle formed by the conductor, the marker, and the reference point isisosceles and, therefore, the depth of the conductor is approximatelyequal to the distance from the reference point to the marker. Thus, nocalculation need be made other than measuring this distance.

Although the invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiment, as well asalternative embodiments of the invention, will become apparent topersons skilled in the art upon reference to the description of theinvention. It is therefore contemplated that such modifications can bemade without departing from the spirit or scope of the present inventionas defined in the appended claims.

I claim:
 1. A differential sensor for an electronic probe, comprising:afirst, generally planar guarded electrode includinga first metallicsensing element, a first metallic shield adjacent said first sensingelement, and first supporting means having opposite faces for supportingsaid first sensing element on one face and said first shield on theopposite face; a second, generally planar guarded electrode includingasecond metallic sensing element, a second metallic shield adjacent saidsecond sensing element, and second supporting means having oppositefaces for supporting said second sensing element on one face and saidsecond shield on the opposite face; and third supporting means forsupporting said first and second guarded electrodes, said first andsecond guarded electrodes being attached to said third supporting meanswhereby said first sensing element on said one face of said firstsupporting means is opposite said second electrode, and said secondsensing element on said one face of said second supporting means ispositioned opposite said first electrode, said first and secondelectrodes lying in different planes.
 2. The device of claim 1wherein:said first guarded electrode further includes a first metallicring at least partially surrounding said first sensing element,supported by said first supporting means; and said second guardedelectrode further includes a second metallic ring at least partiallysurrounding said second sensing element, supported by said secondsupporting means.
 3. The device of claim 1 further comprising:firstenergizing means for supplying an output signal from said first sensingelement to said first metallic shield; and second energizing means forsupplying an output signal from said second sensing element to saidsecond metallic shield.
 4. The device of claim 1 wherein said first andsecond sensing elements are generally parallel with one another.
 5. Thedevice of claim 1 further comprising:first means for amplifying anoutput signal from said first sensing element; and second means foramplifying an output signal from said second sensing element.
 6. Anelectronic probe using the differential sensor of claim 1, and furthercomprising processing means for receiving a first signal from said firstsensing element and a second signal from said second sensing element,and for calculating a signal value based on the difference of said firstand second signals.
 7. A device for determining the direction to anelectrically energized object, comprising:a board member having firstand second sides; first electrode means attached to said board memberproximate said first side thereof, and having a first surface, forgenerating a first signal based upon the electric field potentialcreated by the energized object at said first surface; second electrodemeans attached to said board member proximate said second side thereof,and having a second surface, for generating a second signal based uponthe electric field potential created by the energized object at saidsecond surface; and processing means for receiving said first and secondsignals, and for calculating a signal value based on the difference ofsaid first and second signals.
 8. The device of claim 7 wherein:saidfirst electrode means is guarded and includesa first metallic sensingelement, a first metallic shield adjacent said first sensing element,and first means for supporting said first sensing element and firstshield; and said second electrode means is guarded and includes a secondmetallic sensing element, a second metallic shield adjacent said secondsensing element, and second means for supporting said second sensingelement and second shield.
 9. The device of claim 7 further comprisingmeans, connected to said processing means, for displaying said signalvalue.
 10. The device of claim 7 wherein said processing means includespseudosynchronous detection means for determining the magnitude of saidfirst and second signals.
 11. The device of claim 7 wherein saidprocessing means includes means for attenuating said first and secondsignals.
 12. A circuit tracing system using the device of claim 7,wherein the object is a conductor, and further comprising transmittermeans for applying an alternating voltage to the conductor.
 13. Thesystem of claim 12 wherein:said transmitter means applies saidalternating voltage at a known frequency; and said processing meansincludes pseudosynchronous detection means tuned to said frequency, fordetermining the magnitude of said first and second signals.
 14. Adifferential sensor for an electronic probe, comprising:a first guardedelectrode including a first, generally planar, metallic sensing elementand a first, generally planar, metallic shield attached to and generallyparallel with said first sensing element; a second guarded electrodeincluding a second, generally planar, metallic sensing element and asecond, generally planar, metallic shield adjacent to and generallyparallel with said second sensing element; means for supporting saidfirst and second electrodes such that said first sensing element is on aside of said first shield opposite said second electrode, and saidsecond sensing element is on a side of said second shield opposite saidfirst electrode, whereby said first and second guarded electrodes facein generally opposite directions to increase their combined sensitivity;first energizing means for supplying an output signal from said firstsensing element to said first metallic shield; and second energizingmeans for supplying an output signal from said second sensing element tosaid second metallic shield.